Door knockers and "peep-holes" are popular, especially among urbanites, and the combination of a door knocker and a viewer in a single unit is not new. Representative samples of such can be seen in the following patents, some of which incorporate audio communication in addition to the knocker/viewer.
U.S. Pat. Des. No. 113,884, issued Mar. 21, 1939, for a DOOR KNOCKER; PA1 U.S. Pat. Des. No. 221,130, issued Jul. 13, 1971, for a DOOR MOUNTED PROTECTION UNIT; PA1 U.S. Pat. Des. No. 159,455, issued Jul. 25, 1950, for a DOOR KNOCKER; PA1 U.S. Pat. No. 1,349,731, issued on Aug. 17, 1920, on a COMBINED MAIL OPENING AND DOOR KNOCKER; PA1 U.S. Pat. No. 4,763,444, issued Aug. 15, 1988, for a VIEWING AND ORAL COMMUNICATIONS ASSEMBLY FOR DOORS; PA1 U.S. Pat. No. 1,901,303, issued Mar. 14, 1933, for a ATTACHMENT FOR DOORS; and, PA1 U.S. Pat. No. 2,078,002, issued Jul. 23, 1936, also an ATTACHMENT FOR DOORS.
These units all combine at least two communication functions, such as a knocker and viewer, in a single door-mounted unit.
However, they also share a common drawback in that none effectively disguises the fact that a viewer is incorporated in the structure of the device. Although the caller on the outside of the door may not be able to see through and cannot determine the identity of those inside, or how many people there are, it becomes obvious that someone is home when the viewer is blocked by the shadow of the resident. The normally visible pinhole of light suddenly disappears, as is especially apparent at night.
There are instances when the resident does not wish the person outside to know if anyone is home. Whether it be a pesky neighbor or unpleasant inlaw, or a burglar disguised as a delivery man casing the place, there are many times when the resident might just want to "play possum", preserving his solitude without offending friends and relatives or tipping off felons.
In addition to avoiding callers, some people to are embarrassed to use peepholes and it may be embarrassing to the people that call as well. The peephole makes it clear that the resident does not trust all callers, and there is a natural inclination of the present caller to feel that he is being identified with the group of callers who cannot be entirely trusted.
In the best of all worlds the viewer would be invisible, or combined with another structure and disguised, such that the caller has no way of knowing of its existence.